In the science of geology, there are two main ways we use to describe how old a thing is or how long ago an event took place. When you say that I am 38 years old or that the dinosaurs died out 65 million years ago, or that the solar system formed 4.6 billion years ago, those are absolute ages.
Paleontologists have examined layered sequences of fossil-bearing rocks all over the world, and noted where in those sequences certain fossils appear and disappear.
When you find the same fossils in rocks far away, you know that the sediments those rocks must have been laid down at the same time.
A few days ago, I wrote a post about the basins of the Moon -- a result of a trip down a rabbit hole of book research.
Here's the next step in that journey: the Geologic Time Scales of Earth and the Moon.
If an impact event was large enough, its effects were global in reach.
For example, the Imbrium impact basin on the Moon spread ejecta all over the place.Venus, Io, Europa, Titan, and Triton have a similar problem.On almost all the other solid-surfaced planets in the solar system, impact craters are everywhere. We use craters to establish relative age dates in two ways.The more fossils you find at a location, the more you can fine-tune the relative age of this layer versus that layer.Of course, this only works for rocks that contain abundant fossils.Unfortunately, those methods don't work on all rocks, and they don't work at all if you don't have rocks in the laboratory to age-date. They are descriptions of how one rock or event is older or younger than another.